Minimally invasive surgical cutting instruments are known and widely used to excise and remove biological tissue. Such instruments typically include a handpiece comprising a cutting tool housed within an elongated cannula, wherein the tool is disposed adjacent to an opening at or near the tip of the cannula. The cannula itself is configured for percutaneous insertion into a body via a small incision, and is manually maneuvered into position for tissue excision and removal.
Various configurations of cutting tools are known and may be driven manually, pneumatically or via an electrically controlled drive motor. In any case, tissue adjacent to the opening near the tip of the cannula is typically excised by driving the cutting tool with either a rotary or reciprocal motion relative to the cannula, whereby tissue is drawn into the opening (typically via vacuum) and excised by the cutting tool.
While motor driven surgical cutting instruments of the type just described have been widely used in surgical applications, many presently available designs suffer from a variety of drawbacks. For example, if the position of the cutting tool is not controlled when the drive motor is disabled, there exists a possibility that the cutting tool may come to rest in a position that traps or pinches unexcised tissue between a cutting surface of the cutting tool and the opening near the tip of the cannula. To avoid this problem, surgeons must typically maintain activation of the drive motor as the tip of the instrument is moved or removed from the surgical site, thereby compromising the accuracy and precision of the procedure. The foregoing drawback becomes more problematic as the complexity of the procedure increases, and is of particular concern when performing delicate procedures such as removing vitreous tissue during ocular surgery.
Designers of such surgical cutting instruments have attempted to address the foregoing problem by providing various systems for controlling motor position when stopping or disabling the drive motor. An example of one such system for controlling the position of a three-phase brushless DC motor is given in U.S. Pat. No. 5,602,449 to Krause, et al. The Krause et al. disclosure discloses an elaborate control system including multiple sensors for determining motor armature position at 6.degree. intervals. As is known in the art, brushless DC motors are typically speed driven rather than torque driven and accordingly have little rotational resistance associated with the operation thereof. Controlled stoppage of such a motor is thus extremely difficult, if not impossible, when the motor is operating at a high rotational speed, and the Krause et al. system is accordingly responsive to a motor stop signal to first decrease motor speed below some threshold speed level and then perform a controlled stop based on armature position.
Brushed DC motors, as compared with brushless DC motors, are typically torque driven rather than speed driven, and accordingly have a substantial rotational resistance associated therewith. Thus, while the Krause et al. system may effectively provide for controlled stopping of a surgical cutting instrument driven by a brushless DC motor, such elaborate control techniques are unnecessary when driving a brushed DC motor. What is therefore needed is a simple and inexpensive control technique for controlling the stop position of a brushed DC motor driven surgical instrument. Ideally the control system should be operable to control the position of the cutting tool when the drive motor is turned off so that unexcised tissue is not trapped between the cutting tool and the opening near the tip of the cannula.